The present invention relates to an arc furnace installation with a shroud or enclosure.
The energy problem, the improvements of working conditions and the environmental pollution controls such as noise and dust controlls have recently become severer and the operation of the steel-making arc furnace must be carried out under these severe conditions.
For instance, in order to prevent the atmospheric air pollution by gases exhausted when ores are charged into an arc furnace and molten steel is discharged from it, a ventilation type dust collection system has been employed. That is, the exhaust gases are sucked so that dust particles entrained in the exhaust gases are removed and consequently dust-free gases are discharged into the surrounding atmosphere. However because the ceiling of a steel foundry is high so that smoke and fume are diffused in the air and because the steel foundry has many wide openings, large-sized fans must be installed in order to suck a large quantity of smoke and air as well. Thus the prior art dust collection system has the problems that the system is large in size and the efficiency of the dust collection system is low.
In the case of a large- or intermediate-sized arc furnace, direct suction of exhaust gases is effected during energization. The sucked exhaust gases contain large quantity of CO (carbon monoxide). Therefore, in order to prevent the explosion, a combustion tower or column must be installed to burn CO. This results in generation of large quantities of NO.sub.x (nitrogen oxide).
Meanwhile, noise produced during the operation of an arc furnace reaches as high as 120 dBA so that the operators suffer from occupational diseases; that is, they have difficulty in hearing. In addition, smoke and dust particles discharged from the arc furnace considerably adversely affect the working conditions.
In order to solve these problems, there has been proposed and demonstrated a method for enclosing an arc furnace with a shroud so that it suffices only to suck the exhaust gases from the shroud and the suction of the surrounding air can be minimized because the arc furnace is air-tightly sealed. As a result, as compared with the conventional dust collection systems, the suction rate can be reduced to less than one half and the ventilation or suction system can be reduced in size accordingly. Since the arc furnace is totally enclosed with the shroud, the exhaust gases are burned with the air supplied to the furnace so that the temperature of the exhaust gases drops and the quantities of NO.sub.x are reduced accordingly. In addition, noise can be shut off and the leakage of dust particles to the exterior can be prevented. Thus, the working conditions can be remarkably improved.
When the arc furnace is enclosed with the shroud, the raw materials are once charged by a bucket suspended from a crane into the shroud and then charged into the arc furnace.
Therefore, when an arc furnace is enclosed with a shroud a, the shroud is provided with an opening b as shown in FIG. 1 through which a bucket is moved into or out of the shroud a and which may be closed with doors c. In addition, the roof of the shroud a is provided with a slit-like opening d so that a rope which hangs the bucket may move through the opening d. Therefore, there must be no obstacle which prevents the passage of the rope along the opening d. Furthermore, a sliding door e is provided so as to close the opening d.
Since the shroud a is provided with many openings such as the side and top openings b and d the overall rigidity of the shroud a is considerably decreased so that the shroud a must be constructed with materials having a higher degree of rigidity. As a result, the cost becomes high. Furthermore, in order to air-tightly seal the openings b and d, the shroud a must be constructed with a high degree of precision. For instance, an air curtain must be provided in order to prevent the leakage of smoke from the shroud a, but it is extremely difficult to completely prevent the suction of the surrounding air into the shroud a. As a result, a high-capacity ventilation or suction system must be installed. Furthermore there arises the problem that the operation efficiency is relatively low because the heavy doors c and e must be moved. In addition, an extremely high degree of skill is required for passing the bucketsuspending rope through the slit-like opening d in the roof. If the rope is not correctly moved along the elongated opening d, the bucket suspended therefrom would collide against the shroud a, causing damage to it.
When the scrap or the metal charge is charged, combustibles including impurities are also charged into the arc furnace at high temperature so that the quantities of smoke evolved are increased and smoke is blown up in large quantities above the arc furnace by the ascending air. Consequently, smoke tends to leak out of the shroud.
Meanwhile, toward the end of the melting stage and during the refining stage, exhaust gases at extremely high temperatures are discharged from the arc furnace. Therefore, when such high-temperature exhaust gases are passed through a bag filter, the latter is inevitably damaged. As a result, the high-temperature exhaust gases must be cooled before they enter the bag filter. From the standpoint of energy savings, such cooling systems have been widely used because heat recovered from the exhaust gases may be used to preheat scraps so that the cost of electric power for melting a unit weight of scraps may be reduced and consequently the steel production cost can be decreased accordingly.
The exhaust gases discharged from the arc furnace contain a large quantity of CO which is burned outside the furnace. In the case of a direct suction system, in order to prevent the explosion, a combustion tower or column must be installed to burn CO. After the combustion of CO, the exhaust gases are cooled. Thus, the prior art arc furnace installation needs various equipments.
The operation for preheating the scraps before they are charged into the arc furnace has long been carried out. Since the prior art preheating devices occupy a large installation space, they are installed exterior of and spaced apart from the arc furnace. As a result, the temperature of the preheated scraps drops considerably before they charged into the arc furnace. In addition, the high-temperature scraps are transported in a scrap bucket suspended from a crane while smoke and offensive odor are emitted. Furthermore, cranes for transporting and charging scraps are needed, and a long duct must be extended to the preheating device. Moreover, since the preheating device is installed within the steel foundry, a bypass system or circuit must be installed so that only from 30 to 40% of the exhaust gases may be satisfactorily utilized.
The present invention was made to overcome the above and other problems encountered in the prior art arc furnaces. One of the objects of the present invention is therefore to enclose a preheating chamber for preheating raw materials with a shroud in such a way that the preheating chamber can be moved into or out of the shroud through one side wall thereof in the horizontal direction, whereby the number of openings of the enclosure may be reduced to a minimum and the area of the openings can be also decreased accordingly.
Another object of the present invention is to preheat the raw materials by the exhaust gases discharged from an arc furnace, whereby the problems in the case of the preheated raw materials transported and charged into the furnace can be solved.
The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawings.